1. Field of the Invention
This invention relates to a radiation image recording and read-out apparatus for recording a radiation image of an object on a stimulable phosphor sheet and photoelectrically reading the image recorded on the stimulable phosphor sheet.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in the pattern of the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor and a sheet material bearing thereon a stimulable phosphor layer is referred to as a "stimulable phosphor sheet" and can be used as a recording medium for recording thereon radiation image information.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet comprising the stimulable phosphor is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays which cause it to emit light in proportion to the radiation energy stored. The light emitted from the stimulable phosphor sheet when the sheet is exposed to the stimulating rays is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image having an improved quality, particularly a high diagnostic efficiency and accuracy. The finally obtained visible image may be reproduced in the form of a hard copy or may be displayed on a cathode ray tube (CRT). In this radiation image recording and reproducing system, the stimulable phosphor sheet is used to temporarily store the radiation image in order to reproduce the final visible image therefrom in a final recording medium. For economical reasons, therefore, it is desirable that the stimulable phosphor sheet be used repeatedly.
Further, in a mobile X-ray diagnostic station such as a traveling X-ray diagnostic station in the form of a vehicle like a bus which is provided with a radiation image recording and read-out apparatus for use in the aforesaid radiation image recording and reproducing system and moves from place to place to record radiation images for mass medical examinations, it is disadvantageous to load the mobile X-ray diagnostic station with a large number of stimulable phosphor sheets, and the number of the stimulable phosphor sheets which can be loaded on the mobile X-ray diagnostic station is limited. Therefore, it is desired to load the mobile X-ray diagnostic station with stimulable phosphor sheets which can be used repeatedly, once store the radiation images of the objects in the stimulable phosphor sheets, convey the electric image signals read out from the stimulable phosphor sheets into a recording medium having a large storage capacity, such as a magnetic tape, and circulate and reuse the stimulable phosphor sheets for further image recording and read-out operations, thereby to obtain the radiation image signals of many objects. Further, when image recording is conducted continuously by circulating and reusing the stimulable phosphor sheets, it becomes possible to increase the image recording speed in mass medical examination. This is very advantageous in practical use.
In order to reuse stimulable phosphor sheets as described above, the radiation energy remaining in the stimulable phosphor sheet after it is scanned with stimulating rays to read out the radiation image stored therein should be erased by exposure to light or heat as described, for example, in Japanese Unexamined Patent Publication No. 56(1981)-12599 or U.S. Pat. No. 4,400,619. The stimulable phosphor sheet should then be used again for radiation image recording.
From the aforesaid viewpoint, the applicant proposed in U.S. patent application Ser. No. 600,689 and in Japanese Patent Application No. 58(1983)-66730 a built-in type radiation image recording and read-out apparatus comprising:
(i) a circulating and conveying means for conveying at least one stimulable phosphor sheet for recording a radiation image thereon along a predetermined circulation path, PA0 (ii) an image recording section positioned on said circulation path for recording a radiation transmission image of an object on said stimulable phosphor sheet by exposing said stimulable phosphor sheet to a radiation passing through said object, PA0 (iii) an image read-out section positioned on said circulation path and provided with a stimulating ray source for emitting stimulating rays for scanning said stimulable phosphor sheet carrying said radiation image stored therein in said image recording section, and a photoelectric read-out means for detecting light emitted from said stimulable phosphor sheet scanned with said stimulating rays to obtain an electric image signal, and PA0 (iv) an erasing section for, prior to the next image recording on said stimulable phosphor sheet for which the image read-out has been conducted in said image read-out section, having said stimulable phosphor sheet release the radiation energy remaining in said stimulable phosphor sheet, whereby said stimulable phosphor sheet is circulated through said image recording section, said image read-out section and said erasing section and reused for radiation image recording.
In the aforesaid radiation image recording and readout apparatus, recording and read-out of radiation images can be conducted continuously and efficiently. The radiation image recording and read-out apparatus of this type will be referred to as a "built-in type radiation image recording and read-out apparatus", hereinbelow.
The built-in type radiation image recording and read-out apparatus is advantageous in various points. For example, when recording a number of people at a mobile X-ray diagnostic station such as a bus equipped with an X-ray image recording apparatus, a number of stimulable phosphor sheets equal to the member of people to be diagnosed must be loaded on the bus if the stimulable phosphor sheets are not repeatedly used. However, only a limited number of stimulable phosphor sheets can be loaded on the bus. This problem can be overcome by repeatedly using the stimulable phosphor sheets and staring the image signals read in the image reading section in a recording medium having a large capacity. Further, by circulating the stimulable phosphor sheets, continuous recording is facilitated and the time required to diagnose a given number of people can be shortened.
There has been known tomographic radiography for obtaining a radiation image of a desired cross section of an object and the stimulable phosphor sheet can be used in tomographic radiography for storing the radiation image of the cross section as disclosed in Japanese Unexamined Patent Publication No. 58(1983)-67245. In tomographic radiography, a radiation source, e.g. an X-ray tube, and a stimulable phosphor sheet are opposed to each other with an object intervening therebetween and are moved, upon activation of the X-ray tube, relative to each other substantially satisfying a linear law (wherein the focal point of the X-ray tube, a point in the desired cross section and a point on the stimulable phosphor sheet are linearly aligned). And a geometric law (wherein the ratio of the distance a between the focal point of the X-ray tube and the desired cross section to the distance b between the desired cross section and the stimulable phosphor sheet is constant). As a result, only an image of the desired cross section of the object is focused on the stimulable phosphor sheet and images of other cross sections of the object are made to be out of focus. In tomographic radiography, the linear law and the geometric law have only to be substantially satisfied, and accordingly, the radiation source and the stimulable phosphor sheet may be moved along various orbits including a linear orbit, a circular orbit, an oval orbit, a hypo-cycloid orbit and a spiral orbit.
However, conventional tomographic radiographic apparatuses are disadvantageous in that they are troublesome to operate and are not adapted to continuous recording since a cassette accommodating therein a stimulable phosphor sheet must be set in the tomographic radiographic apparatus each time recording is effected and must be taken out from the apparatus after recording, and then the stimulable phosphor sheet must be taken out of the cassette in order to read the image information stored therein. Therefore, it would be convenient if said built-in type radiation image recording and read-out apparatus were provided with a tomographic recording function so that tomographic recording, reading and erasure are automatically effected.